THE NERVOUS SYSTEM
ham tS.P.Sj. The posterior extremity of the inira-parietal sulcus may bifurcate, and the Jiver^in^ halves constitute the transverse occipital sulcus. The sulcus postcentralis may be quite separate.
The parieto-occipital sulcus tP-O.S. -on^u f parts on the external and mesial surfaces of the hemisphere, and these parts may he equal nr uneuu^. Ti;e external part may be curved or angular. Beddard 175) describes it receiving the simian sulcns in A >i;ii~ang, but that is exceptional. The mesial portion may be very deep, and meet the calearine fissure ». a. S. . Lut this junction is not invariably present.
The simian sulcus .S.S.» sweeps far out over the hemisphere. It is concurrent with the transverse occipital sulcus, and it may enter the parieto-oecipir-jil sulcus.
The calcarine fissure mav be entire as in some lower
Monkeys, or bifid as in most Apes, and accessory sulci may be well developed around it. Its posterior extremity mav be embraced bv the diverging limbs of the
v i o o
Y-shaped lateral occipital sulcus UQ.O.S.*. In the strong development of the latter the brain re>embles that of Monkeys rather than of Apes.
The Sylvian fissure (S.L.) ascends in a slightly sinuous manner, and the parallel sulcus (PA.SJ hooks round its bifurcated caudal end. The anterior part of
the insula (L.O.R.) is exposed at its anterior extremity. Two sulci run forwards from it along the margins of the exposed part of the insula. These sulci do not usually meet, but they did so in a specimen of Hylobates hoolock (fig.oOA). The superior limiting sulcus (S.L. S.*, also known as Marchand's opercular sulcus, runs down-